1. Technical Field
The present invention is directed toward machines for molding objects, and more particularly toward carriages for use in positioning the mold blocks in such machines.
2. Background Art
A well known method for manufacturing corrugated pipes is to extrude a tube of thermoplastic from a head and then conform that tube to the interior of a corrugated mold tunnel formed by mold blocks. The mold blocks typically come in pairs and mate together to form a portion of the mold tunnel, and the thermoplastic is either blown into its shape (by pressure created within the mold tunnel) or by vacuum (where vacuum draws the air from around the mold tunnel).
Structures which have been used in the vacuum method of manufacture are illustrated in U.S. Pat. Nos. 4,319,872, 4,439,130, and 4,718,844. These patents all address the problem of drawing a uniform, strong vacuum around the entirety of the mold blocks which are continually moving during the molding process.
Moving and properly aligning multiple mold blocks is important to ensure that the formed product is continuously uniform along its length without distortions which could occur if the mold blocks are not properly aligned relative to each other. In clamshell type machines, for example (with the mold blocks supported on a single carriage on one side), any forces incurred on the other side of the mold blocks naturally involves a larger moment arm about the carriages, with the result being that such structures can be more susceptible to "sawtooth" type arrangements through the mold tunnel, with successive mold blocks being tilted slightly rather than perfectly aligned with the axis of the mold tunnel.
In order to properly align the mold blocks, it has been found to be advantageous to carry the mold blocks on carriers which themselves have wheels which ride in a track about the path desired for the mold blocks. In order to secure the carriages in the track, the wheels must generally be trapped by opposite sides of the track. In such cases, clearance must be provided (i.e., the opposite track sides must be spaced apart a distance which is greater than the wheel diameters) to allow the wheels to roll on one side without dragging on the other. Such clearance can, however, result in some looseness of the carriage, thereby allowing some degree of sawtoothing. One structure which has been used to address this is shown in U.S. Pat. No. 5,489,201, in which an extra roller is provided along the sides of the carriages, such extra wheel being offset from the other carriage wheels so that it will roll along the top of the machine track while the other carriage wheels roll along the bottom of the machine track.
Rollers have also been provided on carriages and similarly guided in tracks to secure the carriages against lateral shifting during motion. U.S. Pat. No. 4,439,130 shows such a structure. The clearance which must be provided with those rollers (again to prevent the wheel from dragging on one side of the track while rolling on the other side) does, however, potentially allow some degree of undesirable lateral shifting of the carriage and carried mold blocks.
The present invention is directed toward overcoming one or more of the problems set forth above.